docs: add section on ransomware and PBS help against it

docs: add section on ransomware and PBS help against it

Added a section on ransomware. This includes a bullet point in the
main features section and a section in the backup storage section.
The latter section lists mitigation resources in pbs as well as best
practices.

Updated capitalization to be consistent in main features. Imo, since
these are bullet points and not headings, they should be in lowercase

Signed-off-by: Noel Ullreich <n.ullreich@proxmox.com>
Reviewed-by: Stefan Hanreich <s.hanreich@proxmox.com>
Reviewed-by: Stefan Sterz <s.sterz@proxmox.com>